This invention relates to an actuator. The invention is particularly suitable for use in an electrically-operated thrust reverser system, but it will be appreciated that the invention may also be used in other applications.
One type of actuator suitable for use in deploying a thrust reverser cowl utilizes a ball screw mechanism comprising a first element which is rotatable by a motor, the first element cooperating with a second, non-rotatable element such that rotation of the first element causes translation of the second element. The second element is secured in use to the cowl to be moved.
When using low friction screw arrangements of this type to generate axial translation there is a risk that axial loads imposed on the second element may tend to generate rotation of the first element and it is an object of the invention to minimise the risk. Thus according to the present invention there is provided an actuator comprising a first rotatable, element arranged to be driven by a motor in use, a second, axially translatable, element coacting with said first element by way of a low friction screw arrangement whereby rotation of said first element relative to said second element results in axial translation of said second element relative to said first element, and, a brake arrangement applying a braking force to said first element to resist rotation thereof relative to said second element.
It has been found in certain applications that in order to deploy the thrust reverser cowl, the initial force necessary to move the second member must be high, but that movement of the member beyond a predetermined position is possible by applying a lower magnitude force. An actuator has been proposed which is capable of providing an initial, high deploying force phase followed by a phase in which the deploying force applied by the actuator is of reduced magnitude. The actuator comprises a first, rotatable member arranged to be driven by a motor, the first member cooperating with a sleeve such that rotation of the first member causes the sleeve to translate at a low speed through a predetermined distance. Once such translation has been completed, the sleeve rotates with the first member. The sleeve cooperates with a second, non-rotatable member, and rotation of the sleeve causes translation of the second member. The cooperation between the sleeve and the first and second members is such that, for a given speed of rotation of the first member, the sleeve, and hence the second member, initially translates at a low speed, the second member translating at an increased speed when rotation of the sleeve commences.
In use, it is found that once the initial high resistance of deploying movement of the thrust reverser cowl caused by the air flow over the cowl, has been overcome, the air flow tends to assist movement of the cowl. It is desirable to ensure that such assisting or aiding loads or forces are not transmitted back through the actuator as rotational movement, to the motor used to drive the actuator. It is an object of the invention to provide an actuator in which such assisting or aiding loads or forces are not applied to the motor.
According to a second aspect of the present invention there is provided an actuator of the type described hereinbefore, characterised by a brake arrangement applying a braking force to the sleeve, the braking force resisting rotation of the sleeve but not significantly affecting translation of the sleeve.
In such an arrangement, the actuator drive motor does not need to overcome the resistance imposed by the brake arrangement during the initial operating phase in which the actuator extends at a low rate applying a large magnitude force to, for example, a cowl associated with the actuator as the sleeve does not rotate during this phase. However the brake arrangement acts to apply a braking force resisting rotation of the sleeve during the subsequent phase, thereby avoiding or restricting the transmission of any aiding or assisting loads or forces to the motor used to drive the actuator.
The brake arrangement conveniently comprises a brake element rotatable with the sleeve, but slidable longitudinally relative thereto, said element being biased into engagement with a fixed brake surface. It will be appreciated, however, that other brake arrangements are possible. For example, the brake element could be slidably, non-rotatably mounted upon a fixed member and biased into engagement with a surface associated with the sleeve.
Preferably, the cooperation between the sleeve and the first member is achieved using a fine or low pitch ball screw. The cooperation between the sleeve and the second member may be achieved by means of a coarse or higher pitch ball screw. In some circumstances, it is desirable for the actuator to be capable of being reversed by the application of axial load thereto so that rather than simply applying a braking force reducing the speed of rotation of the sleeve, the sleeve is incapable of being rotated by the application of axial loading. Such a result may be achieved by arranging for the cooperation between the sleeve and the second member to take the form of an acme screw thread coupling. In such an arrangement, the brake element may be omitted, the screw threaded cooperation between the sleeve and the second member defining the brake arrangement.